MRI is a diagnostic imaging technique in which image contrasts may be generated. Different image contrasts may be sensitive to different pathologies. Two image contrasts for diagnostic medicine are T1 contrast and T2 contrast. T2 contrast corresponds to the rate at which magnetization disappears perpendicular to the main magnetic field. T2 contrast is useful in clinical imaging. T2 contrast is often used to facilitate identification of tumors, tissue affected by stroke and other lesions, pathologies, and other differences between normal and diseased tissue.
For example, damaged myelin can be distinguished from undamaged myelin based on differences in the T2 decay. One type of damage to myelin, referred to as demyelination, impairs the conduction of signal for an affected neuron. In addition to causing degradation of a neuron's signal, demylenation results in bundle migration. Bundle migration refers to the distance between nerve fibers in a nerve bundle. Some brain diseases including multiple sclerosis (MS), leukoencephalopathy (LE), are related to loss of myelin water (MW). Myelin water fraction (MWF) can serve as a direct indicator of myelin integrity and provide quantitative measurements of myelin structure and change due to white matter diseases.
T2 contrast is typically generated using a spin-echo (SE) or turbo-spin-echo (TSE) sequence. A single spin-echo is formed through a combination of a 90 degree pulse and a 180 degree pulse. The 90 degree pulse generates signal perpendicular to the main magnetic field and is followed by the 180 degree pulse that refocuses transverse magnetization to form an echo. A purely T2 weighted spin echo signal may be acquired at a point in time after the 180 degree pulse. The time at which the echo is acquired, the echo time (TE), occurs at a time after the 180 degree pulse that is equal to the spacing between the 90 degree pulse and the 180 degree pulse.
Conventionally, a T2 decay curve may be measured using a single-slice multi-echo Carr-Purcell-Meiboom-Gill (CPMG) sequence with nonselective composite refocusing pulses and large z-axis gradient crushers. However, the single-slice coverage, long acquisition time (e.g., 25 minutes) due to nonselective refocusing pulses, and increasing signal-to-noise ratio (SNR) are obstacles to practical clinical applications. To increase volume coverage, a conventional multi-slice CPMG sequence with slice-selective refocusing pulse may be used. However, due to intrinsic sensitivity to refocusing imperfection, the measured curve deviates from the true T2 decay curve, which may be pronounced for the first several points.
In another conventional method, a T2 preparation spiral imaging sequence is used. However, this method yielded MWF maps with low resolution and poor image quality. Multi-slice techniques based on T2* signal decay have also been proposed to achieve larger volume coverage and shorter acquisition time. However, high sensitivity to local field inhomogeneity is a limitation of this technique for practical applications since local field gradient along a slice direction can introduce non-exponential signal decay. Furthermore, a signal void region is observed in MWF maps even when high-order shim is performed to minimize field inhomogeneity before data acquisition and the correction of field inhomogeneity is performed in the post-processing.